SERBAN JOURNA OF EECTRCA ENGNEERNG Vol. 6, No. 1, May 2009, 75-88 UDK: 621.311.1.015.1 More Stability and Robustness with the Multi-loop Control Solution or Dynamic Voltage Restorer (DVR) Othmane Abdelkhalek, Abderrahmane Kechich, Tarek Benslimane, Chellali Benachaiba, Mohammed Haidas 1 Abstract: This paper presents the application o dynamic voltage restorers (DVR) on power distribution systems or mitigation o voltage sags/swells at critical loads. DVR is one o the compensating types o custom power devices. An adequate modeling and simulation o DVR, including controls in MATAB, based on orced-commutated voltage source converter (VSC), has been proved suitable or compensating the voltage sags/swells. n this paper, a double loop control method is proposed or the improvement o the stability o DVR during the load current variation. For the main loop (Outer Voltage oop), we use a controller or the regulation o the ilter condenser voltage. The second loop (nner Current oop) also uses a controller to control the disturbances current during load variation. Simulation results are presented to illustrate and understand the perormances o DVR in the compensation o voltage sags/swell even with variation load condition. Keywords: Custom power, ower quality, Voltage sags, Voltage swells, Current controller, Voltage controller, WM, DVR. 1 ntroduction Modern power systems are complex networks where hundreds o generating stations and thousands o load centers are interconnected through long power transmission and distribution networks [1]. The main concern o consumers is the quality and reliability o power supplies at various load centers where they are located. Even though the power generation in most welldeveloped countries is airly reliable, the quality o the supply is not entirely reliable. ower distribution systems, ideally, should provide their customers with an uninterrupted low o energy at smooth sinusoidal voltage at the contracted magnitude level and requency [2]. However, in practice, power systems, especially the distribution systems, have numerous nonlinear loads, which signiicantly aect the quality o power supplies. As a result o the 1 Department o Electrical and Engineering, University o Bechar, Algeria; E-mail: othmane08000@yahoo.r 75
O. Abdelkhalek, A. Kechich, T. Benslimane, C. Benachaiba, M. Haidas nonlinear loads, the purity o the waveorm o supplies is lost. This ends up producing many power-quality problems. Apart rom nonlinear loads, some system events, both regular (e.g. condenser switching, motor starting) and iregular (e.g. aults) could also inlict power quality problems [3]. A power quality problem is deined as any problem maniested in voltage/current change or requency deviations that result in ailure or malunctioning o a customer s equipment [3,4]. ower quality problems are associated with an enormous number o electromagnetic phenomena in power systems with broad ranges o time rames such as long duration variations, short duration variations and other disturbances. Short duration variations are mainly caused by either ault conditions or energising o heavy loads that require high starting currents. Dierent impedance-related electrical distances and types o grounding and connection o transormers between the aulted/load location and the node may induce a temporary loss o voltage or temporary voltage reduction (sag) or voltage rise (swell) at dierent nodes o the system [5]. Voltage sag is deined as a sudden plummet in voltage supply rom 90% to 10% o nominally, ollowed by a recovery ater a short period o time. A typical duration o the sag is l0 ms to 1 minute, according to the standard. Voltage sag can induce the production loss in automated processes since voltage sag can trip a motor or cause the controller to malunction. Voltage swell, on the other hand, is deined as a soar in voltage supply rom 110% to 180% in rms voltage at the network undamental requency, its duration being rom 10 ms to 1 minute. Switching o a large inductive load or energising a large condenser bank is a typical system event that causes swells [1]. To compensate the voltage sag/swell in a power distribution system, appropriate devices need to be installed at adequate locations. These devices are typically installed at the point o common coupling (CC) deined as the spot o change o the network ownership. 2 Dynamic Voltage Restorers A DVR is a device that injects dynamically controlled voltage Vinj () t in series to the bus voltage by means o a transormer as depicted in Fig. 1. There are three single-phase transormers connected to a three phase converter with energy storage system and control circuit [5]. The amplitudes o the three injected phase voltages are controlled so as to eliminate any detrimental eects o a bus ault to the load voltage V () t. This means that any dierential voltage caused by transient disturbances in the ac eeder will be compensated by an 76
More stability and robustness with the Multi-loop control solution or Dynamic... equivalent voltage generated by the converter and injected on the medium voltage level through the booster transormer. The DVR operates independently o the type o ault or any event occuring in the system, provided that the whole system remains connected to the supply grid, i.e. the line breaker does not trip. For most practical cases, a more economical design can be achieved by only compensating the positive and negative sequence components o the voltage disturbance seen at the input o the DVR. This option is reasonable owing to typical distribution network coniguration; the zero sequence part o a disturbance will not pass through the step down transormers owing to ininite impedance or this component. For the most part DVR does nothing except monitoring the bus voltage, which means that it does not inject any voltage ( Vinj () t = 0) independent o the load current. Thereore, we recommend that the attention should be ocused particularly on the losses o a DVR during normal operation. Two speciic eatures addressing this loss issue have been implemented in its design, the transormer design including low impedance, and the semiconductor devices used or switching. An equivalent circuit diagram o the DVR and the principle o series injection or sag compensation are depicted in Fig. 2. Fig. 1 Schematic diagram o DVR System. Fig. 2 Equivalent circuit o DVR. 77
O. Abdelkhalek, A. Kechich, T. Benslimane, C. Benachaiba, M. Haidas Fig. 3 Compensation strategy o DVR or voltage sags. Mathematically expressed, the injection satisies: V( t) = VS ( t) + Vinj( t), (1) where V () t is the load voltage, VS ( t ) is sagged supply voltage and Vinj ( t ) is the voltage injected by the mitigation device as shown in Fig. 3. Under nominal voltage conditions, the load power on each phase is given by (2): * S = V = jq, (2) where is the load current, and, and Q are the active and reactive power taken by the load respectively during a sag/swell. When the mitigation device is active and restores the voltages back to normal, the ollowing applies to each phase: S = jq = jq + jq, (3) ( ) ( ) S S inj inj where sag subscript reers to the sagged supply quantities. The inject subscript reers to quantities injected by the mitigation device. 3 Control Strategy For the identiication o disturbance, we use the ark transormation technique (dq-rame) [2-4]. This work is primarily ocused on closed-loop WM control scheme (Fig. 4). The closed-loop control introduced in this paper is made up o an inner current loop and an outer voltage loop to better satisy dierent linear or nonlinear load disturbance. System level simulations o the whole inverter and control system will be perormed based on MATAB-Simulink. roportionalplus-integral () controller will be used in this dual-loop control system to regulate output voltage o the WM inverter. 78
More stability and robustness with the Multi-loop control solution or Dynamic... Fig. 4 Dual-loop control scheme or single-phase WM inverter. The current and voltage dual-loop control system or a single-phase WM inverter can eectively reduce the output voltage distortion and achieve ast dynamic response [7]. n this control scheme output voltage V C and inductor current will orm an outer loop and an inner loop each governed by a controller. n additional, load current load and the output voltage V C will act as eedback compensation or the reduction o output disturbance even under rough load conditions. The reerence input will give the desired output voltage as a control reerence. To determine K and K values o both regulators, we study every loop independent o each other. Fig. 5 shows the control loop block diagram, where all the components are represented by their respective transer unctions or gains. n particular, the controller block is represented by the typical proportional integral regulator structure, whose parameters K and K will be determined in the ollowing. The output o the regulator represents the modulating signal that drives the pulse width modulator. This has been modeled as the cascade combination o two separate blocks: the irst one is the modulator static gain, and the second is actually a irst-order ade approximation o its delay, considered equal to a hal o the duration o the modulation period. Considering the inverter and load models, we see that they based on the analysis which is presented as ollows. Finally, to ully replicate a typical implementation, a transducer gain is taken into account. Additional ilters, normally adopted to clean the transducer signal rom residual switching noise, are not taken into account, in avor o a more essential presentation. Their transer unctions can be easily cascaded to the transducer block gain i needed. 79
O. Abdelkhalek, A. Kechich, T. Benslimane, C. Benachaiba, M. Haidas 3.1 nner current loop and design o the controller At irst, we want to determine the open loop gain or the block diagram o Fig. 5. This is given by the cascade connection o all blocks. We ind TS 1 s K 2V DC 1 () 4 GT GO t = K + s C T K S 1+ s R 1+ s 4 R. (4) The regulator design is typically driven by speciications concerning the required closed loop speed o response or, equivalently, the maximum allowed tracking error with respect to the reerence signal. These speciications can be turned into equivalent speciications or the closed loop bandwidth and phase margin. To give an example, we suppose that, or our current controller, a closed loop bandwidth,, equal to about one sixth o the switching requency S is required, to be achieved with, at least, a 60 phase margin, phm. We thereore have to determine the parameters guarantee the compliance to these requirements. K and K so as to To rapidly get an estimation o the searched values, we suppose that we can approximate the open loop gain at the crossover angular requency, i.e. at ω=ω = 2π with the ollowing expression: G ( jω ) K O TS 1 jω 2V DC 4 GT 1 C T K S R 1+ jω 4 1+ jω R. (5) This, in principle, will be a good approximation as long as K <<ω K (to be veriied later). mposing now the magnitude o (5) to be equal to one at the desired crossover requency, we get K C R K = 1+ ω 2V G R DC T 2. (6) The parameter K can then be calculated considering the open loop phase margin and imposing that to be equal to phm. We ind rom (4) 80
More stability and robustness with the Multi-loop control solution or Dynamic... 0 0 1 TS 1 1 K 180 + phm = 90 2 tan ω tan tan 4 ω + ω, (7) R K which yields K ωk =. (8) 0 1 TS 1 tan 90 + phm + 2tan ω + tan ω 4 R Fig. 5 Control loop block diagram. Note that (8) is exact; only the K value is obtained through an approximation. Considering the parameters listed in Table 1 and 2π S -1 ω = 52.4krads, we can immediately ind the ollowing values: 6 4-1 K = 6.284 and K = 1.802 10 rads. t is easy to veriy that the condition K <<ω K is reasonably met by this solution. This occurs in vast majority o practical cases, so that (6) and (8) can be very oten directly used. 3.2 Outer voltage loop and design o the controller The voltage controller gains can be determined once the desired loop bandwidth,, is speciied. For a DVR application, in order to achieve a satisactory control o the voltage waveorm in the presence o distorting loads. While this is easy to obtain when the switching requency is relatively high, as it is in our case, and the current controller is a ast one, like the one we are considering here, in the opposite case, i.e., when a low switching requency 81
O. Abdelkhalek, A. Kechich, T. Benslimane, C. Benachaiba, M. Haidas application is considered or when the internal control loop is relatively slow, it may not be too easy to achieve the desired values. However, once is known, we can consider the open loop gain expression and orce its magnitude to be equal to one at the desired crossover requency. From Fig. 6 the open loop gain is ound to be: GTV 1 sts 1 GO V =. (9) GT 1+ st K S sc K + s t is worth noting that, dierently rom the current controller case, no delay eect related to the holder has been taken into account. This is possible because the internal current control loop has been designed to compensate or that. Thereore, the only dynamic delay the voltage controller has to compensate is that o the current controller. Given (9), the irst condition we need to satisy by suitably choosing K and K is as ollows: 2 G K ( ) 2 ω TV K = 1. (10) 2 GT ωc where, as usual, ω = 2π. The second constraint we can impose is requiring a minimum phase margin, phm, or the loop gain at the crossover requency. n order to get a reasonable damping o the dynamic response, this can be set equal to 60. Consequently, we ind the ollowing additional condition: 0 0 1 1 K 180 + phm = 180 2 tan ( ω TS ) + tan ω. (11) K The solution o the system o equations (10) and (11), considering the parameter values listed in Table 1 and imposing = 1800Hz provides us 3-1 with the ollowing values or the gains: K = 3.81 and K = 3.42 10 rads. Fig. 6 Control loop block diagram. 82
More stability and robustness with the Multi-loop control solution or Dynamic... 4 Simulation Results Table 1 arameters. Filter inductance = 1.5 mh Filter resistance R = 0.002 Ω Filter condenser C = 68μ F hase supply voltage DC link voltage Supply requency WM carrier peak Switching requency V = 110 Vrms S V = 250 V DC 60 Hz C = 4V K = 50 khz Current transducer gain 1 = 0.1VA S G T Voltage transducer gain 1 = 0.02 VV G TV Transormer rapport 1:1 hase margin phm = 60 n order to understand the perormance o the DVR along with control, a simple power supply network is presented in Fig. 7. A DVR is connected to the system through a series transormer with a rapport transormation equal to 1:1. The DVR is based on three phase voltage WM inverter with C output ilter to remove high requency voltage components. An R- load ( R = 10Ω, 6 = 10 H ) is considered. First, a case o symmetrical sag is simulated by connecting and disconnecting a three-phase reactance to the busbar. The results are shown in Fig. 8a 30% voltage sag is initiated at 0.05s and it is kept until 0.15s. Figs. 9b and 9c show the voltage components injected by the DVR and compensated load voltage, respectively. As a result o DVR use, the load voltage is kept at 110 Vrms. n order to show the perormance o the DVR under critical conditions, a phase (a) o supply voltage outage at 0.052 s is simulated. The DVR is immediately compensating the voltage phase absence. The results are shown in Fig. 9. t can be seen, rom the results, that the DVR is able to produce the required compensating voltage components or dierent phases rapidly and help to maintain a balanced and constant load voltage at 110 Vrms. 83
O. Abdelkhalek, A. Kechich, T. Benslimane, C. Benachaiba, M. Haidas Fig. 7 DVR coupling in power system. 84
More stability and robustness with the Multi-loop control solution or Dynamic... Fig. 8 Simulation result o DVR response to a balanced voltage sag with connection and disconnection load: (a) supply voltage, (b) DVR injected voltage, (c) load voltage, (d) load current. Fig. 9 Simulation result o DVR response to a phase voltage outage with connection and disconnection load: a) supply voltage, b) DVR injected voltage, c) load voltage, d) load current. 85
O. Abdelkhalek, A. Kechich, T. Benslimane, C. Benachaiba, M. Haidas Fig. 10 Simulation result o DVR response to balance voltage swill with connection and disconnection load: a) supply voltage, b) DVR injected voltage, c) load voltage, d) load current. Fig. 11 Simulation result o DVR response to unbalance voltage deormation with connection and disconnection load: a) supply voltage, b) DVR injected voltage, c) load voltage, d) load current. 86
More stability and robustness with the Multi-loop control solution or Dynamic... Next, the perormance o DVR or a voltage swell condition is investigated. The results are shown in Fig. 10a. The voltage amplitude is increased about l50% o nominal voltage. The injected voltage that is produced by DVR in order to correct the load voltage and this ormer are shown in Figs. 10b and 10c, respectively. As can be seen rom the results, the load voltage is kept at the nominal value. Similar to the case o voltage sag, the DVR reacts quickly to inject the appropriate voltage component to correct the supply voltage. The perormance o the DVR with an unbalanced voltage deormation is shown in Fig. 11. The compensating voltage injected by the DVR is shown in Fig. 11b and the load voltage is given in Fig. 11c. Notice the constant and balanced voltage across the load. n this part, we tested the DVR robustness in the critical condition due to the load current prompt variations. This robustness is tested using a double-loop control technique that ensures the control at the same time o voltage and DVR injected current. The irst loop named Outer Voltage oop, controls through a regulator the voltage across o the condenser C. The second loop, named nner Current oop, controls through also a regulator the current lowing in the inductance with load current prompt variation. n all the above Figs. 8d, 9d, 10d and 11d the load current variation (disconnection and connection o the load) does not inluence the stability o compensating injected DVR voltage. 5 Conclusion n this paper, perormance o a DVR in mitigating voltage sags/swells is demonstrated with the help o MATAB. The DVR handles both balanced and unbalanced situations without any diiculties and injects the appropriate voltage component to correct any anomaly in the supply voltage to keep the load voltage balanced and constant at the nominal value. n the case o voltage sag, which is a condition o a temporary reduction in supply voltage, the DVR injects an equal positive voltage component in all three phases, which are in phase with the supply voltage to correct it. On the other hand, or a voltage swell case, which is a condition o a temporary increase in supply voltage, the DVR injects an equal negative voltage in all three phases, which are anti-phase with the supply voltage. For unbalanced conditions, the DVR injects an appropriate unbalanced three-phase voltage components positive or negative depending on whether the condition is an unbalanced voltage sag or unbalanced voltage swell. The proposed control solution is the most important issue o this paper. The current and voltage dual-loop control system or a WM inverter can eectively reduce output voltage distortion and achieve ast dynamic response. 87
O. Abdelkhalek, A. Kechich, T. Benslimane, C. Benachaiba, M. Haidas 6 Reerences [1] S.W. Wahab, A.M. Yuso: Voltage Sag and Mitigation using Dynamic Voltage Restorer (DVR) System, Elektrika, Vol. 8, No. 2, 2006, pp. 32-37. [2] M.A. Taghikhani, A. Kazemi: A New hase Advanced Multiloop Control System For Dynamic Voltage Restorer, nternational Journal o Emerging Electric ower Systems, Vol. 3, No. 2, 2005. [3] M.R. Banaei, S.H. Hosseini, S. Khanmohamadi, G.B. Gharehpetian: Veriication o a New Energy Control Strategy or Dynamic Voltage Restorer by Simulation, Simulation Modelling ractice and Theory, Vol. 14, No. 2, 2006, pp. 112-125. [4] E. Babaei, S.H. Hosseini, G.B. Gharehpetian, M.T. Haque, M. Sabahi: Reduction o DC Voltage Sources and Switches in Asymmetrical Multilevel Converters using a Novel Topology, Electric ower Systems Research, Vol. 77, No. 8, 2007, pp. 1073-1085. [5] K. ee, K. Yamaguchi, H. Koizumi, K. Kurokawa: D-UFC as a Voltage Regulator in the Distribution System, roceedings o Renewable Energy Conerence, 2006. [6]. Boonchiam, N. Mithulananthan: Understanding o Dynamic Voltage Restorers Through MATAB Simulation, Thammasat nternational Journal o Science and Technology, Vol. 11, No. 3, 2006, pp. 1-6. [7] S.. Jung, H.S. Huang, M.Y. Chang, Y.Y. Tzou: DS-based Multiple-loop Control Strategy or Single-phase nverters used in AC ower Sources, ower Electronics Specialists Conerence ESC '97 Record, 28th Annual EEE, Vol. 1, 1997, pp. 706-712. 88